Andreas Rossberg wrote:

>The latter also provides a good argument against making physical equality
>too polymorphic. It would break abstraction, much worse than it does
>already. In particular, a program's meaning could depend on implementation
>details (like false being represented by 0) in very questionable ways.
>  
>
It's arguable that it wouldn't be /that/ much worse than the current 
situation, where == on non-mutable structures is already implementation 
dependent.
I am not advocating for such a change though, if only because in most 
cases the arguments of == are known to be of the same type, so it is a 
good thing to be able to take advantage of this information. However, it 
would still be nice to have both equalities available (yes, I know, yet 
another equality). In the problem I'm working on, only mutable 
structures are compared, so the extended == would work as intended. Of 
course, you would have to know what you're doing when using this 
predicate, but this is already the case for the current two equalities : 
you have to watch out for the aforementioned non-mutable structures with 
== and for cyclic values with = .

Call me pessimistic, but I have little hope that this "feature wish" 
will be fulfilled, so I would happily settle for fun a b -> a == 
(Obj.magic b) (if it works, still haven't had a chance to try it). I'd 
be grateful if someone could tell me whether this particular use of 
Obj.magic is safe.

Christophe


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